ct: Add ability to kill container w/o pid namespace

For that the LIBCT_OPT_KILLABLE should be set, then
killing would go via service cgroup.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
test: Add test for killing container w/o pid namespace

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>